1. Field of the Invention
The present invention relates generally to a communication system. In particular, the present invention relates to a signal receiving apparatus and method in a MIMO (Multiple-Input Multiple-Output) Communication System.
2. Description of the Related Art
Typically, a transmitted radio-frequency signal is distorted due to a variety of factors including multipath interference shadowing, propagation attenuation, time-varying noise, and interference in the radio channel environment of mobile communication systems, in comparison to signals transmitted in the wired channel environment. Fading caused by the multipath interference is closely related to reflectors or the mobility of a user, that is, a user terminal, and the actual transmitted signal that is mixed with the interference signal in a received signal. Thus, the received signal is a transmitted signal with severe distortion, which degrades the whole performance of the mobile communication system. Consequently, since the fading phenomenon may distort the amplitude and phase of the received signal, it is a major factor that hinders high-speed data communication in the radio frequency channel environment. Hence, signal fading has been an active area of study. High-speed data transmission in the mobile communication system requires minimizing the losses inherent to radio channels such as fading, and interference between users. As an approach to preventing fading-incurred unstable communications, diversity schemes are used. One of many diversity schemes is space diversity scheme that uses multiple antennas. The space diversity scheme is branched into a receive diversity, a transmit antenna diversity, and a MIMO. The receive antenna diversity is achieved by use of a plurality of receive antennas and the transmit antenna diversity is achieved by use of a plurality of transmit antennas. MIMO is a scheme for using a plurality of receive antennas and a plurality of transmit antennas.
A MIMO communication system uses multiple antennas in each transmitter (such as Node B) and a receiver (such as user equipment (UE)). The MIMO scheme is suitable for transmission of a large amount of data in a future-generation mobile communication system. Thus, the MIMO scheme is being actively studied. If the channels between the transmit antennas and the receive antennas are independent identically distributed (IID) ones, the number of transmit and receive antennas M are equal, and the bandwidth and transmit power are constant, the MIMO scheme has an average channel capacity about M times as large as that of a Single-Input Single-Output (SISO) scheme, thereby bringing a high gain in channel capacity. Use of the same spreading code for different transmit antennas in the MIMO communication system enables code reuse and, as a result, produces a gain in the data rate as compared to the SISO scheme.
A major example of a MIMO communication system is a Per-Antenna Rate Control (PARC) communication system. With reference to FIGS. 1 and 2, the PARC communication system will now be described.
FIG. 1 is a block diagram of a transmitter in a typical PARC communication system.
Before describing FIG. 1, it is assumed that the transmitter uses M transmit antennas and J spreading codes, wherein M and J are both integers. Thus, the transmitter can transmit M×J user data streams simultaneously.
Referring to FIG. 1, a user data stream to be transmitted is provided first to a demultiplexer (DEMUX) 100. The DEMUX 100 demultiplexes the user data stream in correspondence with the number of the transmit antennas, M. Specifically, the DEMUX 100 separates the user data stream into M user data streams and provides the first to Mth user data streams sequentially to the first to the Mth signal processors 110 to 114 in a one-to-one correspondence.
The first to Mth signal processors 110 to 114 each include an encoder, an interleaver and a modulator. They encode their input user data streams according to a predetermined coding method that interleaves the coded data according to a predetermined interleaving method, and modulates the interleaved data according to predetermined modulation method. The outputs of the first to Mth signal processors 110 to 114 each are provided to corresponding spreaders in first to jth spreading units 120 to 124. Each spreading unit includes M spreaders. That is, the first spreading unit 120 includes M spreaders, which receive signals from the first to Mth signal processors 110 to 114, respectively, multiply the received signals by a first spreading code (SC1), and provide the products to first to Mth adders 130 to 134, respectively. In this way, the Jth spreading unit 124 includes M spreaders, which receive signals from the first to Mth signal processors 110 to 114, respectively, multiply the received signals by a Jth spreading code (SCJ), and provide the products to the first to Mth adders 130 to 134, respectively. Consequently, the spread signals that were processed in the same signal processor are output to the same adder.
The first to Mth adders 130 to 134 add the received signals and output them to the first to the Mth transmit antennas 140 to 144 (Tx. ANT 1 to Tx. ANT M), respectively.
While not shown in FIG. 1, the outputs of the first to Mth adders 130 to 134 are subject to additional signal processing including scrambling, digital-to-analog conversion, and filtering to be transmittable over the air, before being provided to the transmit antennas 140 to 144. Let the signals fed to the transmit antennas 140 to 144 be denoted by s1(t), s2(t), . . . , sM(t).
As described above, the PARC communication system spreads a user data stream with a plurality of spreading codes and transmits the spread signals through transmit antennas. The plurality of spreading codes are applied commonly to each of the transmit antennas, thereby achieving effective code reuse and increasing resource efficiency.
The MIMO communication system that is being studied only takes into account a flat fading channel environment. Hence, many PARC receiver structures have only been proposed for the flat fading channel environment. The proposed PARC receiver structures include Minimum Mean Square Error (MMSE) and Successive Interference Cancellation (MMSE-SIC), which is a combination of MMSE and SIC.
Now a description will be made of the SIC scheme.
Typically, interference cancellation (IC) is a scheme for generating an interference signal and canceling the interference signal at a receiver. Here, the interference signal is considered to be the remaining signal of a received signal except for the desired signal to be detected.
There are two IC schemes: SIC and Parallel Interference Cancellation (PIC). The SIC scheme is further divided into decision-feedback and Bell Labs Layered Space-Time (BLAST). In the SIC scheme, signals are detected from a received signal in a descending order of signal strength. Specifically, the strongest signal, that is, the most interfering signal is first cancelled from the received signal using its hard-decision value. Then the next most interfering signal is cancelled from the remaining signal using its hard-decision value. By repeating this procedure, a final desired signal is detected.
As described above, the SIC scheme relies much upon the previous estimation. If errors are involved in the previous estimation, interference increases significantly, degrading performance. That is, as errors are generated in the estimate of a stronger signal, the performance degradation becomes more serious.
A variety of methods for solving the problem have been proposed. Among them, there are Partial SIC (PSIC), parallel detection, and sphere detection. According to the PSIC scheme, only a fraction of the strongest signal is cancelled from a received signal. If the ratio of the strongest signal is 1, the ratio of the fraction is between 0 and 1. The parallel detection scheme applies Maximum Likelihood (ML) Detection only to the transmit antenna that has transmitted the strongest signal, thereby increasing the reliability of the strongest signal. The sphere detection scheme generates a sphere having an appropriate radius from the most probable closes point on a constellation and estimates symbols within the sphere by ML detection.
For application to a PARC receiver, the MMSE-SIC performs better than the MMSE. Now, the PARC receiver will be described with reference to FIG. 2.
FIG. 2 is a block diagram of a receiver in the typical PARC communication system.
Before describing FIG. 2, it is assumed that the receiver uses N receive antennas and J spreading codes. While the number of the transmit antennas may be equal to that of the receive antennas, it is assumed that they are different as illustrated in FIG. 2. Referring to FIG. 2, first to Nth receive antennas 200 to 204 (Rx. ANT 1 to Rx. ANT N) each receive signals from all the transmit antennas 140 to 144 illustrated in FIG. 1.
The first to Nth receive antennas 200 to 204 provide the received signals to their corresponding despreaders 220 to 228. Specifically, the first receive antenna 200 outputs its received signal to (1-1)th to (I-J)th spreaders 220 to 222, the second receive antenna 202 outputs its received signal to (2-1)th to (2-J)th spreaders 223 to 225, and so on in this manner, until the Nth receive antenna 204 outputs its received signal to (N,1)th to (N,J)th spreaders 226 to 228, respectively.
The (1-1)th to (1-J)th despreaders 220 to 222 despread the signal received from the first receive antenna 200 with the same J spreading codes as used in the transmitter and output the despread signals to first to Jth MMSE receivers 230 to 234, respectively. Specifically, the (1-1)th despreader 220 despreads the received signal with SC1 and outputs the despread signal to the first MMSE receiver 230, and the (1-2)th despreader 221 despreads the received signal with SC2 and outputs the despread signal to the second MMSE receiver 232. In the same manner, the (1-J)th despreader 222 despreads the received signal with SCJ and outputs the despread signal to the Jth MMSE receiver 234.
The (2-1)th to (2-J)th despreaders 223 to 225 despread the signal received from the second receive antenna 202 with the same J spreading codes as used in the transmitter and output the despread signals to the first to Jth MMSE receivers 230 to 234, respectively. Specifically, the (2-1)th despreader 223 despreads the received signal with SC1 and outputs the despread signal to the first MMSE receiver 230, and the (2-2)th despreader 224 despreads the received signal with SC2 and outputs the despread signal to the second MMSE receiver 232. In the same manner, the (2-J)th despreader 225 despreads the received signal with SCJ and outputs the despread signal to the Jth MMSE receiver 234.
In this way, the (N-1)th to (N-J)th despreaders 226 to 228 despread the signal received from the Nth receive antenna 204 with the same J spreading codes as used in the transmitter and output the despread signals to the first to Jth MMSE receivers 230 to 234, respectively. Specifically, the (N,1)th despreader 226 despreads the received signal with SC1 and outputs the despread signal to the first MMSE receiver 230, and the (N-2)th despreader 227 despreads the received signal with SC2 and outputs the despread signal to the second MMSE receiver 232. In the same manner, the (N-J)th despreader 228 despreads the received signal with SCJ and outputs the despread signal to the Jth MMSE receiver 234.
The first to Jth MMSE receivers 234 each detect a corresponding user data stream from the received signals by MMSE. Specifically, the first MMSE receiver 230 detects a user data stream from the signals received from the (1-1)th, (2-1)th, . . . , (N-1)th despreaders 220, 223, . . . , 226 by MMSE, the second MMSE receiver 232 detects a user data stream from the signals received from the (1-2)th, (2-2)th, . . . , (N-2)th despreaders 221, 224, . . . , 227 by MMSE, and in the same manner, the Jth MMSE receiver 234 detects a user data stream from the signals received from the (I-J)th, (2-J)th, . . . , (N-J)th despreaders 222, 225, . . . , 228 by MMSE. Consequently, each of the first to Jth MMSE receivers 230 to 234, respectively, receives the signals despread with the same spreading code.
A multiplexer (MUX) 240 multiplexes the signals received from the first to Jth MMSE receivers 230 to 234, respectively. A signal processor 250, including a decoder, a deinterleaver, and a demodulator, decodes the multiplexed signal in a decoding method corresponding to the coding, deinterleaves the decoded signal in a deinterleaving method corresponding to the interleaving, and demodulates the deinterleaved signal in a demodulation method corresponding to the modulation in the transmitter. It is assumed herein that the signal processor 250 sequentially detects the user data streams transmitted from the first to Mth transmit antennas 140 to 144, respectively. The signal processor 250 outputs a user data stream having the highest signal strength to a signal reproducer 260. The signal strength is a measure of Signal to Interference and Noise Ratio (SINR). For conciseness, it is assumed that the user data streams are stronger in the order of the Mth transmit antenna 144< . . . < the second transmit antenna 142<the first transmit antenna 140.
The signal reproducer 260 reconstructs an original transmitted signal by processing the signal from the signal processor 250, that is, the user data stream transmitted by the first transmit antenna 140 in the manner of the signal processing in the transmitter, and outputs the reconstructed signal to first to Nth subtractors 210 to 214. The signal reproducer 260 includes an encoder, an interleaver, and a modulator, which encodes the received signal in a predetermined coding method, interleaves the coded signal in a predetermined interleaving method, and modulates the interleaved signal in the modulation method of the transmitter.
The first subtractor 210 subtracts the reproduced signal from the signal received through the first receive antenna 200 and outputs the resulting signal to the (1-1)th to (1-J)th despreaders 220 to 222. The second subtractor 212 subtracts the reproduced signal from the signal received through the second receive antenna 200 and outputs the resulting signal to the (2-1)th to (2-J)th despreaders 223 to 225, respectively. In the same manner, the Nth subtractor 214 subtracts the reproduced signal from the signal received through the Nth receive antenna 204 and outputs the resulting signal to the (N-1)th to (N-J)th despreaders 226 to 228, respectively.
The above operation is repeatedly performed on the strongest to weakest user data streams. Thus, the receiver accurately detects the user data stream transmitted by the transmitter, while sequentially reducing the effects of the multiple transmit antennas in the PARC communication system.
Meanwhile, the real radio channel environment is similar to a frequency-selective fading channel environment, and a spatial channel model (SCM) under consideration in the MIMO communication system takes into account six paths. As described earlier, however, the current study of the MIMO communication system is confined to the fading channel environment. Therefore, there is a need for a MIMO communication system that is implemented under the real radio channel environment such as the frequency-selective fading channel environment.